In offset printing presses, in particular web-fed offset rotary printing presses, the images to be printed are transferred by a form cylinder onto an offset cylinder, which is usually referred to as a rubber blanket cylinder. The image is then transferred from the rubber blanket cylinder onto a printing material. In the past, printing blankets consisting of a multi-layer construction, which are commonly referred to as rubber blankets, were predominantly clamped on the rubber blanket cylinder with the ends of the rubber blanket fixed in the cylinder channel. This type of rubber blanket typically has a thickness of approximately 2 mm. Accordingly, an opening of at least 4 mm was necessary just to insert the two ends of the rubber blanket into the cylinder channel. As a rule, the width of the clamping channel of a rubber-covered cylinder had to be substantially larger due to various factors in order to be able to change the rubber blankets during operation without disturbances.
A rubber blanket, in which the flexible layers are arranged on a rigid flexible carrier is disclosed in WO 01/70 512 A1. In particular, the rubber layers of the blanket have corresponding reinforcements comprising a textile, a covering layer (i.e., a printing layer) and customarily a compressible layer. The carrier is customarily a metal plate. For reasons of cost, a steel plate is frequently used. The rubber-layer construction is typically fixed to the steel plate, particularly adhesively bonded to the steel plate. As described, this construction comprises at least one covering layer whose surface holds the printing image. This surface can consist of, for example, an elastomeric material. It is advantageous, but not necessarily required, to provide a woven-fabric layer below the covering layer. A layer of compressible material that can contain air inclusions is customarily used below the woven-fabric layer. This layer construction can be fixed to the carrier plate by an adhesion promoter. The leading and trailing ends of the plate are not covered by the rubber-layer construction. As a result, only the thin steel sheets have to be inserted into the cylinder channel and fastened in the channel. The result of this is a substantial reduction in the channel width, which has substantial advantages, as is known. In the rubber blanket according to WO 01/70512 A1, the ends of the rubber layer are therefore opposite one another above the cylinder channel.
A printing blanket, in which the rubber layer, like the printing blanket described above also has a layer construction consisting of a plurality of layers is disclosed in DE 101 17 409 A1. This layer construction is sealed at the two ends of the printing blanket. The sealant extends over the overall height of the material thickness of the rubber-blanket-layer construction. Moreover, as a result of its approximately triangular cross section, the sealant partially covers the free ends of the metal layer (i.e. of the sheet-metal plate) that are to be inserted into the cylinder channel. This arrangement prevents, in particular, fluids used during the printing process and also washing fluids from penetrating into the woven-fabric inlay or penetrating below the uppermost layer and impairing the intimate assembly of the layer construction. In this known rubber blanket, it is considered advantageous for the thickness of the elastomeric coating sealant to be increased continuously in the direction of the metal plate. As a result, a significantly large area of the elastomeric coating is produced above the rubber-covered plate or above an adhesion-promoter layer used on the rubber-covered plate. This measure is intended to counteract tangential forces that are produced, in particular, at the base point of the printing blanket, namely in the transition region of the elastomeric coating or of the sealant.
The known metallic backed rubber blankets described above have the disadvantage that their service life is considerably less than the service life of conventional rubber blankets not having a metal plate. However, the sealing of the printing blanket surface as far as the steel sheet described above ensures complete sealing.